The majority of bus systems available on the market comprise a central bus module that converts the mains AC voltage into a DC voltage and applies this DC voltage to the power supply lines of the bus system. From there, the DC voltage is tapped by the individual subscriber devices of the bus system and is converted into an AC voltage that is fed to a transformer or converter for then being re-converted to a DC voltage, and if necessary regulated, for powering the electronic part of the subscriber device. If a transformer is used, the subscriber devices are galvanically isolated from the central power supply, thereby meeting a requirement often demanded in the application of such devices. On the other hand, in case of many subscribers the total cost and complexity of the communication system is significant, because similar device components have to be provided for each of the subscribers. Subscriber devices usually have an outer housing with an interior that provides only limited space for device components. This is one of the reasons why eliminating device components inside the housing of subscriber devices is desirable.
Thus, the prior art communication system is problematic for such aspects as:                space required for connecting an electronic module within a device housing;        costs and complexity for galvanic isolation within the subscriber device as required according to electrical safety guidelines; or        costs and complexity for power supply to the electronic modules.        
DE 198 44 394 B4 discloses a two-conductor bus system for data transfer between stations having transmission and reception circuits, which applies an AC voltage to the bus which is picked up by a power tap and is provided to the station. For data transmission, DC components are superimposed on the AC voltage, with a DC frequency that is smaller than the frequency of the AC voltage.
EP 0 355 532 A2 discloses an arrangement for transfer of data and of a rectified supply voltage via a pair of bus lines, in which each of the subscriber devices is connected to the pair of bus lines via a transformer. The data signals are fed into and extracted from the pair of bus lines via coil-capacitor circuits.
DE 102 16 330 B4 discloses a measuring device for process technology including a bus system in which a central unit feeds a plurality of modules and in which a feedback unit is provided which influences the supply voltage.
DE 10 2009 003 846 A1 discloses a system for non-contact data and power supply to bus subscriber modules that can be arranged side by side on a mounting base. The mounting base comprises a mounting rail and a supply bar, the latter comprising primary-side flat coils which cooperate with shorter secondary-side flat coils of bus subscriber modules. In this way, the bus subscriber modules can be arranged along the supply bar at any location and do not have to rely on latching points of the mounting rail. The primary-side flat coil is supplied with energy and data signals by a gateway configuration, namely inductively via respective coils. From the bus level which presumably includes lines there exists a bus level interface to a gateway module.
DE 10 2010 016 865 A1 discloses a mounting rail bus system comprising a mounting rail and a station bus disposed therein which consists of interconnected bus members. Module-like bus devices can be connected to conductive traces, by contact pins and by a circuit board, for transferring electrical data signals and/or control signals.
DE 198 13 364 A1 discloses a power supply for a control system which comprises a master station and decentralized stations. The master station is powered with AC voltage from the mains which is known to provide 50 or 60 Hz, via a transformer. Two-wire data lines and power supply lines run from the master station to the decentralized stations. There, presumably, the AC voltage is converted into DC voltage for powering electronics in the decentralized stations. The space required for the power supply of the electronic modules within the decentralized stations is considerable.